Magnetic resonance imaging is a technique for performing imaging using the phenomenon of magnetic resonance. The principles of magnetic resonance imaging mainly include: in an atomic nucleus containing a single proton, such as the hydrogen nuclei which are present throughout the human body, the proton thereof has spin motion and so resembles a small magnet. The spin axes of these small magnets have no fixed pattern; if an external magnetic field is applied, the small magnets will realign in accordance with the lines of magnetic force, specifically aligning in a direction parallel to a line of magnetic force of the external magnetic field, or in a direction antiparallel to a line of magnetic force of the external magnetic field. The direction parallel to a line of magnetic force of the external magnetic field is called the positive longitudinal axis, while the direction antiparallel to a line of magnetic force of the external magnetic field is called the negative longitudinal axis; a nucleus has only a longitudinal component of magnetization, this longitudinal component of magnetization having both a direction and a magnitude. Nuclei in the external magnetic field are excited using a Radio Frequency (RF) pulse of a specific frequency, such that the spin axes of these nuclei deviate from the positive longitudinal axis or negative longitudinal axis, resulting in resonance; this is the phenomenon of magnetic resonance. Once the spin axes of the excited nuclei have deviated from the positive longitudinal axis or negative longitudinal axis, the nuclei have a transverse component of magnetization.
Once emission of the RF pulse has stopped, the excited nuclei emit an echo signal, gradually releasing the absorbed energy in the form of an electromagnetic wave, and the phase and energy level thereof return to their pre-excitation states; by subjecting the echo signals emitted by the nuclei to further processing, such as spatial encoding, an image can be reconstructed.
FIGS. 1a to 1c show schematic diagrams of a type of multi-slab image encoding based on a 3D fast spin echo sequence. FIG. 1a is a schematic diagram showing partitioning of sub-slabs; FIG. 1b is a schematic diagram showing partial encoding of the sub-slabs; FIG. 1c is a schematic diagram showing the relationship between the excited layer thickness and expanded layer thickness for each sub-slab.
As FIG. 1a shows, each slab of the imaging region is first divided into multiple sub-slabs in the slice direction; the case of 8 sub-slabs is taken as an example in FIG. 1a. An imaging scan is then performed on each sub-slab using 3D fast spin echoes.
One slice encoding step during specific encoding and imaging is shown in FIG. 1b, in which RF, SL (Slice), PE (Phase Encoding), RO (Readout) and ADC (analog-digital converter) correspond to radio frequency pulse, slice selection gradient, encoding gradient, readout gradient and data acquisition module, respectively. The method comprises: emitting a sequence of pulses of different angles within a repetition time TR, at the same time varying the phase encoding gradient with a certain slice encoding gradient, so as to fill one slice encoded k-space; within another repetition time TR, the RF pulses remain unchanged and the slice encoding gradient is changed, giving another slice encoded k-space; and so on until data for the whole k-space is collected. In the pulse sequence, one 90 degree selective exciting pulse is applied first, a slice selection gradient corresponding to the current sub-slab being applied in the SL direction at the same time. One al-degree selective inverting RF pulse is then emitted; at the same time, a slice selection gradient corresponding to the current sub-slab and a slice encoding gradient are applied in the SL direction, a first encoding gradient is applied in the PE direction, and the ADC is then used to perform data acquisition. One a2-degree selective inverting RF pulse is then emitted; at the same time, a slice selection gradient corresponding to the current sub-slab and a slice encoding gradient are applied in the SL direction, a second encoding gradient is applied in the PE direction, and the ADC is then used to perform data acquisition, and so on until data for the whole k-space is collected.
During the imaging process, expansion must be performed in accordance with a predetermined expansion factor on either side of the sub-slab slice direction, so as to obtain a slice encoded thickness greater than the excited thickness, encoding being performed on the slice thickness corresponding to this encoded thickness. As shown in FIG. 1c, the thickness TH corresponding to the middle shaded region of FIG. 1c is the excited thickness of the current sub-slab, while the thickness STH corresponding to the whole region of FIG. 1c is the slice encoded thickness of the current sub-slab. The excited thickness is generally equal to the thickness of the corresponding sub-slab. The expansion factors corresponding to each sub-slab are generally equal.
Since a metal implant (MI) may be implanted inside a living body for the purpose of securing or replacing a joint or other vital tissue during orthopedic surgery and other emergency operations, in practical applications the presence of a metal insert will give rise to inhomogeneity in the external magnetic field, leading to geometric distortion of the image. For each sub-slab, this geometric distortion is mainly embodied in slice deformation of the excited sub-slab, the slice deformation corresponding to an excited sub-slab being different for different distances between the excited sub-slab and the metal implant. In the schematic diagram of FIG. 2, showing the positions of sub-slabs relative to a metal implant MI, the slice deformation of the nth sub-slab, which is closer to the metal implant MI, is greater than that of the mth sub-slab, which is remote from the metal implant MI.
Furthermore, different types of metal implant give rise to different slice deformations in an excited sub-slab. If the same expansion factor is used to expand the encoded thickness for each sub-slab, full acquisition of the image data arising from the slice deformation of each excited sub-slab is not possible, so the distorted image cannot be restored fully during the image reconstruction stage.